One object seen in two contrary motions at once
The needle-hole and straw experiment demonstrating true and false motion
The page opens with the claim that a single pupil can see one object make two contrary motions at the same instant, without any alteration of the pupil. A pinhole-vision diagram follows: an eye at left (labelled q, b, o, a), a pierced card in the middle, and the wall at right marked P, r/n, S. The text prescribes the experiment: look at the air through a needle-hole in paper with the eye held close, then move a very thin straw between eye and hole from right to left. On the near side of the hole the straw is seen moving truly, and beyond the hole it is seen moving in reverse, so the true and false motions appear simultaneously and separated from one another.
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One pupil sees two contrary motions at the same time
Leonardo states as a proposition that a single pupil can see one object make, at one and the same moment, two contrary motions, and that this happens without any alteration of the pupil itself.
The needle-hole and straw experiment (labels q b o a, r n p, q m, S c)
Look at the air through a tiny hole made in paper by a needle's point, holding the eye close, and interpose a very thin straw between eye and hole. Moved from right to left, the straw is seen in true motion between the hole and the eye, in its real place; beyond the hole it is seen moving contrary to its true motion, so true and false motion are seen at once.